INTRODUCTION For a variable air volume system@ it is typical to modulate supply fan speed to maintain a duct static pressure set point. Traditionally@ this set point is a constant aiming to ensure proper air distribution under design load (ASHRAE 1995). This set point is the summation of the total pressure loss along the air duct downstream of the sensor and the terminal box pressure required by the manufacturer under design conditions. However@ under partial load conditions@ the terminal box dampers will close to reduce airflow. Since the pressure loss is proportional to the square of the airflow ratio@ the required pressure set point can be reduced. Using constant pressure set point at partial load condition leads to more fan power consumption due to higher fan head. What's more@ with higher average duct pressure@ air leakage in ducts will increase. This also results in more fan power consumption. The supply fan control method using static pressure reset can significantly reduce fan power. Liu (2007a) has demonstrated a simple fan power savings model by comparing constant static pressure set point and static pressure reset. It is also demonstrated that without static pressure reset@ the design minimum airflow for pressure dependent box can not be achieved due to higher pressure before the terminal box dampers. Liu et al. (1997b) studied the impact of low static pressure in dual-duct systems on fan energy consumption. Besides the theoretical research@ fan power savings due to static pressure reset is demonstrated by experiments and case studies. The impact of static air pressure on the fan power was recognized by Warren and Norford (1993). The static air pressure reset schedule was investigated by Rose and Kopko (1994). Significant energy savings and improved indoor comfort conditions have been measured and presented by Claridge et al. (1996). Liu et al. (1995) presented the impacts of VFD and static pressure reduction on energy consumption. Even though the fan power savings potential due to static pressure reset has been widely acknowledged@ no mathematical model has been developed to quantitatively analyze its impacts@ especially with the consideration of air leakage. Also@ the resulted thermal energy savings have not yet been demonstrated. An air leakage rate at 10 to 20% of the inlet airflow rate in commercial building is not uncommon (Fisk et al. 2000)@ and it has great impacts on both fan power and thermal energy consumption. Wray (2003) has demonstrated the increase in annual fan energy is estimated to be 40 to 50% for a system with a total leakage of 19% at design conditions compared to a tight system with 5% leakage rate. Annual cooling plant energy also increases by about 7 to 10%. However@ his study in a VAV system only consider the case with constant static pressure setpoint. This paper presents mathematical models to demonstrate the impacts of static pressure reset on air leakage@ fan power and thermal energy for both pressure independent and pressure dependent boxes. Air leakage reduction is considered as an important factor contributing to the fan power and thermal energy savings.